1. Field of the Invention
The present invention relates to a process for producing a semiconductor substrate, particularly a silicon-on-insulator (hereinafter referred to as SOI) substrate having a silicon dioxide film (hereinafter referred to as a buried oxide film) located inside a semiconductor silicon substrate, formed by implanting oxygen ions into the semiconductor silicon substrate and then heat-treating the substrate.
2. Description of the Related Art
Processes of forming a buried oxide film in a semiconductor silicon substrate by implanting oxygen ions into the semiconductor silicon substrate and then heating the substrate, are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-188,239, 3-240, 230, and 4-264,724, U.S. Pat. No. 4,676,841, and J. Mater. Res., vol. 8 (1993), pp. 523-534. The heat treatment temperature, time, and atmosphere are disclosed in 1990 IEEE SOS/SOI TECHNOLOGY CONFERENCE PROCEEDINGS, pages 45, 47, and 48 and also in Materials Science and Engineering, B12(1992), pp. 27-36 and pp. 41-45. The disclosed heat treatment atmospheres are nitrogen gas alone at normal pressure, argon gas alone, a mixture of nitrogen gas and 0.5% to 1% oxygen gas, and a mixture of argon gas and 0.5% to 1% oxygen gas.
FIGS. 1(a) to 1(d) and 2(a) to 2(c) show a conventional process of forming a buried oxide film in a semiconductor silicon substrate.
FIGS. 1(a) to 1(d) represent semiconductor silicon substrate having an oxygen atom density of less than 4.0.times.10.sup.22 atoms/cm.sup.3 established by oxygen ion implantation.
Referring to FIG. 1(a), oxygen ions 2 are implanted into a semiconductor silicon substrate 1 through one surface thereof, for example, at a dose of from 0.3.times.10.sup.18 ions/cm.sup.2 to 0.4.times.10.sup.18 ions/cm.sup.2 and an ion implantation energy of from 150 keV to 220 keV.
Referring to FIG. 1(b), the semiconductor silicon substrate 1 with implanted oxygen ions has, in the inside thereof, a high oxygen concentration layer 3 and a high defect density layer 4 containing atomic vacancies or other crystal defects generated by the ion implantation.
Referring to FIG. 1(c), the semiconductor silicon substrate 1 is then heat-treated, for example, at a temperature of 1300.degree. C. or higher in an atmosphere of argon gas mixed with 0.5% oxygen gas for a time of about from 15 minutes to 1 hour, with the result that the implanted oxygen precipitates to form a silicon oxide layer 5 in the region of the high oxygen concentration layer 3 and also forms silicon oxide islands 6 in the region of the high defect density layer 4.
Referring to FIG. 1(d), the heat treatment is continued for further 2 hours to 6 hours with the result that the silicon oxide islands 6 disappear by being incorporated in the silicon oxide layer 5, as the former are disadvantageous in free energy, while the silicon oxide layer 5 grows to form a buried oxide film 7 with a semiconductor silicon film 8 formed thereon.
FIGS. 2(a) to 2(c) represent semiconductor silicon substrates having an oxygen atom density of 4.0.times.10.sup.22 atoms/cm.sup.3 or more established by oxygen ion implantation.
Referring to FIG. 2(a), oxygen ions 2 are implanted into a semiconductor silicon substrate 1 through one surface thereof, for example, at a dose of from 1.25.times.10.sup.18 ions/cm.sup.2 to 2.2.times.10.sup.18 ions/cm.sup.2 and an ion implantation energy of from 150 keV to 220 keV.
Referring to FIG. 2(b), the semiconductor silicon substrate 1 implanted with the oxygen ions has, in the inside thereof, a silicon oxide layer 5. A high defect density layer containing crystal defects is generated during the ion implantation, but disappears by being overlapped with the silicon oxide layer 5 upon completion of the ion implantation.
Referring to FIG. 2(c), the semiconductor silicon substrate 1 is then heat-treated, for example, at a temperature of 1300.degree. C. or higher in an atmosphere of argon gas mixed with 0.5% oxygen gas for a time of about from 2 hours to 1 hour, with the result that the silicon oxide layer 5 grows to form a buried oxide film 7 with a semiconductor silicon film 8 formed thereon.
The SOI substrate produced by the above-mentioned conventional process has the following problems. One problem is that the buried oxide film contains microdefects lacking in oxygen atoms due to spatial fluctuation of the precipitation, because the buried oxide layer is formed by the precipitation of the ion-implanted oxygen atoms and grows by Ostwald ripening of the precipitate. Nucl. Inste. Meth. B84 (1994), page 270 describes that the microdefects lacking in oxygen atoms may provide a path for current leakage or may substantially reduce the dielectric breakdown strength of the buried oxide film in comparison with a thermal silicon oxide film. Moreover, the interface between the buried oxide film and the adjoining silicon layers has a roughness ten times or more than the roughness of the interface between a thermal oxide film and the adjoining silicon layers obtained by thermally oxidizing a mirror-finished silicon substrate as used in the usual semiconductor production process.
Another problem of the conventional processes is that the oxygen ion dose, i.e., the amount of implanting oxygen ions per unit area is limited to an extremely small range in order to provide an SOI substrate having a buried oxide film and surface silicon layer of good quality, as can be seen from J. Mater, Res. vol. 8 (1993), pp. 523-534 or Mater. Sci. Eng. B12 (1992), pp. 41-45. Therefore, in the above literature, the obtained buried oxide film has an extremely limited thickness range of about from 80 to 90 nm.
Thus, the buried oxide film 7 obtained by the conventional process contains a substantial amount of the microdefects lacking in oxygen atoms and providing a path for electric current leakage and also has a large roughness of the interface between the buried oxide layer and the adjoining semiconductor silicon film 8. Moreover, the buried oxide film obtained by the conventional process has a substantially fixed thickness (in cm) determined by dividing the amount of implanting oxygen ions (in ions/cm.sup.2) by 4.48.times.10.sup.22.
Therefore, when the SOI substrate produced by the conventional process is used to fabricate an integrated circuit, the current leakage and fluctuation in threshold values of the transistors leads to the problems that the product yield of the IC chips is reduced and that the IC design is materially limited by the thickness of the buried oxide film.